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Question
Two lenses A and B have focal lengths of +20 cm and, −10 cm, respectively.
(a) What is the nature of lens A and lens B?
(b) What is the power of lens A and lens B?
(c) What is the power of combination if lenses A and B are held close together?
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Solution
(a) The focal length of lens A is +20 cm. The positive sign indicates that lens A is convex, i.e., a converging lens.
The focal length of lens B is -10 cm. The negative sign indicates that lens B is concave, i.e., a diverging lens.
(b) Focal length of lens A, fA = +20 cm = +0.20 m
∴ Power of lens `A, P_A=1/(f_A)=1/0.20=5D`
Focal length of lens B, fB = -10 cm = -0.10 m
∴ Power of lens B, `P_B=1/f_b=1/-0.10=-10D`
(c) When lenses are combined, the power of the combination is the algebraic sum of the powers of the individual lenses.
∴ Power of combination of lens A and B,p=P_A=P_B
` P=5D-10D=-5D.`
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